The purpose of pre-press proofs, as is well known in the art, is to assess color balance and strength which can be expected from the final press run and accordingly to correct the separation transparencies before the printing plates are made therefrom. In many instances it is also required to produce so-called customer proofs for approval of subject, composition and general appearance of the print prior to press run. Thus it is essential that the pre-press proof should have the same appearance as the press print, that is to say in addition to matching the colors of the press print, the pre-press proof should be on the same paper as the press print.
On the basis of the pre-press proofs, the color separation transparencies are accepted or corrected if necessary and then used for the preparation of printing plates. There are so-called positive working and negative working printing plates, as is well known in the art. A positive working printing plate is exposed to a positive transparency or film positive wherein the information to be printed corresponds directly to opaque areas, whereas the non-printing background areas correspond to transparent areas contained on such film positive. By exposing such positive working plate to light through a film positive, the exposed areas contained thereon are rendered removable by chemical treatment and the underlying usually grained aluminum plate surface then forms the water receptive nonprinting or non-image areas, whereas the unexposed areas contained thereon form the ink receptive printing or image areas during the subsequent lithographic or offset printing.
A negative working printing plate is exposed to light through a film negative wherein the information to be printed corresponds to transparent areas, whereas the non-printing background areas correspond to opaque areas contained on such film negative. In this case the exposed areas become photo-hardened and form the ink receptive printing areas whereas the unexposed areas are removed by chemical treatment and the underlying water receptive usually grained aluminium plate surface forms the non-printing or non-image areas during subsequent lithographic or offset printing.
It is known to produce by electrophotographic processes lithographic and gravure pre-press proofs containing in general four colors, such as yellow, magenta, cyan and black. Such pre-press proofing processes are disclosed for instance in U.S. Pat. Nos. 3,337,340, 3,419,411 and 3,862,848.
It is customary to produce such electrophotographic pre-press proofs by charging a photoconductive recording member followed by exposure through a separation film positive corresponding to one color, followed by toning of the exposed photoconductor with a liquid toner of the appropriate color, followed by in-register transfer of the color toned image deposit to a receiving member surface, such as paper, usually of the same grade as the printing stock. These process steps are then repeated with separation film positives of the other three or more colors and appropriate color toners to produce a multi-color pre-press proof of print as required.
It should be noted that prior art electrophotographic pre-press proofing processes are so-called direct reproduction processes, that is to say the color separation transparencies employed include film positives wherein the image areas to be reproduced correspond directly to the opaque image areas on such film positives. Consequently in such prior art electrophotographic pre-press proofing processes the latent image formed on the photoconductor upon exposure to such positive separation films is developed by attracting thereto liquid toner material of opposite polarity to that of the electrostatic charges constituting said latent images whereby the so formed toner deposits on the photoconductor surface correspond directly to the image areas to be reproduced. Thus prior art electrophotographic pre-press proofing processes are employed only for proofing of film positives which are used for the preparation of positive working printing plates.
Prior art electrophotographic pre-press proofing processes are not suitable for the proofing of film negatives used for the preparation of negative working printing plates, in that such processes are not suitable for the reversal reproduction of imagery wherein the transparent areas contained on a film negative are to be reproduced as the image areas on the pre-press proof. Reversal reproduction per se by electrophotography is well known in the art but the processes employed for this purpose are not suitable for multicolor pre-press proofing.
Reversal image reproduction in electrophotography is normally carried out according to prior art practices by means of so-called repulsion toning. This process includes the steps of electrostatically charging the surface of a photoconductor to a polarity, typically charging an n-type photoconductor such as zinc oxide to negative polarity, exposing the surface to a film negative containing the image to be reproduced in the form of transparent areas and the non-image part in the form of opaque areas whereby the photoconductor surface becomes discharged in the exposed image areas while retaining the charge in the unexposed non-image areas and applying to the surface toner material having the same polarity as that of the charges contained on the surface, typically applying negative toner material to a negatively charged n-type photoconductor surface, whereby such toner material is repelled from the charged non-image areas onto the discharged image areas forming toner deposits thereon corresponding to the image to be reproduced. The thus formed image deposits in certain instances are fused to the photoconductor surface whereas in other instances they are transferred to a receptor sheet.
Such above described image reversal reproduction by electrophotography is very well suited to microfilm and microfiche reproduction and reader/printers wherein the information to be reproduced generally is in the form of alphanumeric characters and lines and where complete fill-in of large solid areas and complete absence of fog or stain in the non-image areas are not absolutely required. In pre-press proofing however in order to match the image quality of the press print sheet it is essential to have on the pre-press proof large solid areas completely filled in and background areas completely free of fog or stain. These requirements cannot be met by the prior art electrophotographic reversal process, because unlike by attraction toning, by repulsion toning it is not possible to produce uniformly filled in large solid areas. This is because toner repulsion from a charged background area onto an uncharged solid image area is most effective near the edges of the solid area where the intensity of the field lines from the charged background area terminating in the uncharged image area is highest. The intensity diminishes in effectiveness towards the center of the solid image area where the intensity of the terminating field lines is lowest. This results in solid image areas characterized by high density near the edges and a so-called hollow or lower density center. For the same reason, in repulsion toning the background non-image areas are completely free of fog or stain only near the edges. This so-called edge effect cannot be fully overcome even by using biasing devices during repulsion toning, that is by placing a so-called developing electrode a short distance apart from the photoconductor surface to thereby enhance toner deposition as is well known in the art.
A color proofing apparatus and method is disclosed in U.S. Pat. No. 4,556,309 ('309) whereby multicolor proofs are produced from positive color separation films only. In the method taught in the '309 patent, a photoconductor plate is uniformly charged, exposed to light through a positive transparency, toned in the image areas and the toned image deposits are transferred to an offset member and then to a receptor, such as a suitable grade of paper. In view of the above-identified advantages of producing proofs using negative transparencies and attraction toning, it would be desirable to be able to adapt a conventional proofer similar to that disclosed in the '309 patent, designed to use the positive color separation process, to convert to a negative color separation process without requiring significant changes to the proofer apparatus.